dt-bindings: timer: hpe,gxp-timer: Add HPE GXP Timer and Watchdog

Add support for the HPE GXP Timer and Watchdog. There are multiple
timers on the SoC but only one is enabled at this time.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>